The

goldfish


is


classified

as

a

coldwater


fish],


and

can

live


in


unheated

aquarium
at

a

temperature


comfortable


for


humans.


However,


rapid


changes


in
temperature

(for

example


in


an

office


building


in


winter


when


the


heat


is


turned
off

at


night)


can

kill


them,


especially


if

the


tank


is


small.


Care

must


also


be

taken
when

adding


water,


as

the


new


water


may


be

of


a

different


temperature.
Temperatures

under

about


10

°C

(50


°F)


are


dangerous


to


fancy


varieties,


though

commons


and

comets


can
survive
slightly

lower


temperatures.


Extremely


high


temperatures


(over


30

°C

(86


°F)


can

also


harm

goldfish.
However,

higher


temperatures


may


help


fight


protozoan


infestations

by

accelerating


the


parasite's

life

-

cycle—
thus

eliminating


it

more


quickly.


The


optimum


temperature


for


goldfish


is


between


20

°C

(68


°F)


and

22

°C
(72

°F).
Like

all


fish,


goldfish


do

not


like


to


be

petted.

In


fact,


touching


a

goldfish


can

endanger


its

health,


because


it
can
cause

the


protective


slime


coat


to


be

damaged

or


removed,


exposing


the


fish’s


skin


to


infection


from
bacteria
or


water

-

born


parasites.


However,


goldfish


respond

to


people


by

surfacing

at


feeding


time,


and

can
be
trained

or


acclimated


to


taking

pellets


or


flakes

from

human

fingers.


The


reputation


of


goldfish


dying


quickly
is

often


due

to


poor


care.
If
left


in


the


dark


for


a

period


of


time,


goldfish


gradually

change


color


until


they


are


almost


gray.


Goldfish
produce
pigment

in


response

to


light,


in


a

similar


manner


to


how


human

skin


becomes

tanned

in


the


sun.


Fish
have
cells


called

chromatophores


that


produce

pigments

which

reflect


light,


and

give


the


fish

coloration.


The
color

of


a

goldfish


is


determined


by

which

pigments

are


in


the


cells,


how


many


pigment

molecules


there


are,
and
whether

the


pigment

is


grouped

inside

the


cell


or


is


spaced


throughout


the


cytoplasm.
Because

goldfish


eat


live


plants,


their


presence

in


a

planted


aquarium

can

be

problematic.


Only

a

few
aquarium
plant


species


for


example


Cryptocoryne


and

Anubias,


can

survive

around

goldfish,


but


they


require
special

attention


so

that


they


are


not


uprooted.


Plastic


plants

are


often


more


durable,


but


the


branches

can
irritate

or


harm

a

fish

that


touches


one.
Feeding
In

the


wild,


the


diet


of


goldfish


consists

of


crustaceans,


insects,


and

various


plant


matter.


Like


most


fish,


they
are

opportunistic

feeders


and

do

not


stop


eating]


on

their


own


accord.


Overfeeding


can

be

fatal,


typically

by
blocking
the


intestines.


This


happens


most


often


with


selectively

bred


goldfish,


which

have

a

convoluted
intestinal

tract.


When

excess

food


is


available,


they


produce

more


waste

and

faeces,


partly


due

to


incomplete
protein
digestion.


Overfeeding


can

sometimes


be

diagnosed

by

observing


faeces


trailing


from

the


fish's
cloaca.
Goldfish-
specific


food


has

less


protein

and

more


carbohydrate


than


conventional


fish

food.


It

is


sold


in


two
consistencies—flakes
that


float,


and

pellets


that


sink.

Enthusiasts

may


supplement


this

diet


with


shelled

peas
(with
outer


skins


removed),


blanched

green

leafy

vegetables,

and

bloodworms.


Young


goldfish


benefit


from
the

addition


of


brine


shrimp


to


their


diet.


As

with


all


animals,


goldfish


preferences


vary.
Behavior
Behavior

can

vary


widely


both


because


goldfish


live


in


a

variety


of


environments,


and

because


their


behavior
can
be

conditioned

by

their


owners.